Work machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy machinery use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are selectively fluidly connected to a pump on the work machine that provides pressurized fluid to chambers within the actuators, and to a tank to allow the pressurized fluid to drain from the actuators. A valve arrangement is typically fluidly connected between the actuators and the pump and tank to control a flow rate and direction of pressurized fluid to and from the chambers of the actuators.
The portion of the valve arrangement connecting the actuator to the tank is called a drain valve. The drain valve typically includes a solenoid operated electronic flow controlling valve or a hydraulic pressure limiting valve. The electronic flow controlling valve has a valve element that is movable against a spring bias between a flow-passing and a flow-blocking position in response to an electronic signal to control a flow of pressurized fluid to an actuator. The hydraulic pressure limiting valve generally includes a valve element that is spring biased toward a flow-blocking position and movable toward a flow-passing position in response to a fluid pressure exerted against the valve element to limit a maximum pressure within the actuator.
A system having one of the electronic flow controlling and hydraulic pressure limiting valves can be problematic, while a valve arrangement having both the electronic flow controlling and hydraulic pressure limiting valves can be large and expensive. For example, the hydraulic pressure limiting valve does not afford the controllability of the electronic flow controlling valve, while the electronic flow controlling valve can not afford pressure limiting functions during electrical failure or system shut down and is not as responsive as the hydraulic pressure limiting valve. One method of providing the benefits of both the electronic flow controlling and hydraulic pressure limiting valves is described in U.S. Pat. No. 5,878,647 (the '647 patent) issued to Wilke et al. on Mar. 9, 1999. The '647 patent describes a hydraulic circuit having two pairs of valves, a variable displacement pump, a reservoir tank, and a hydraulic actuator. One pair of the valves includes a head-end supply valve and a head-end return valve that connects a head end of the hydraulic actuator to either the variable displacement pump or the reservoir tank. The other pair of solenoid valves includes a rod-end supply valve and a rod-end return valve that connects a rod end of the hydraulic actuator to either the variable displacement pump or the reservoir tank. Each of the head and rod-end return valves includes a solenoid operated pilot valve element that selectively communicates fluid from the hydraulic actuator to a hydraulically operated valve element. When both the solenoid operated pilot valve element and the hydraulically operated valve element are in a flow-passing position, fluid from the hydraulic actuator is allowed to drain from the hydraulic actuator to the reservoir tank.
Although the return valves of the hydraulic circuit described in the '647 patent may provide some of the benefits associated with both electronic flow controlling and hydraulic pressure limiting valves, the return valves of the '647 patent may still be problematic. For example, in the situation of electrical failure or system shut down, the return valves of the '647 patent do not perform any pressure limiting functions. Further, because flow through the return valves can be completely blocked by high fluid pressures acting on the hydraulically operated valve element, the hydraulic circuit of the '647 patent lacks control. In addition, excessive pressures within the hydraulic circuit of the '647 patent tend to move the hydraulically operated valve element toward a flow-blocking position rather than a flow-passing position, thereby allowing the excessive pressures to increase even further.
The disclosed valve is directed to overcoming one or more of the problems set forth above.